Heart failure refers to the heart's inability to keep up with the demands made upon it. Congestive heart failure refers to an inability of the heart to pump an adequate amount of blood to the body tissues. Because the heart is unable to pump an adequate amount of blood, blood returning to the heart becomes congested in the venous system.
In a healthy heart, the heart pumps all of the blood that returns to it, according to the Frank-Starling law. Increased venous return leads to increased end diastolic volume, which causes increased strength of contraction and increased stroke volume. In addition to intrinsic control according to the Frank-Starling law, a healthy heart is subject to extrinsic control, such as stimulation by the sympathetic nervous system to enhance contractility.
In a patient experiencing congestive heart failure, intrinsic and extrinsic control mechanisms may not function properly, and consequently the heart may fail to pump an adequate amount of blood. Failure of the left side of the heart is generally more serious than the failure of the right side. On the left side of the heart, blood returns from the pulmonary system and is pumped to the rest of the body. When the left side of the heart fails, there are consequences to both the pulmonary system and to the rest of the body. A patient with congestive heart failure may be unable to pump enough blood forward to provide an adequate flow of blood to his kidneys, for example, causing him to retain excess water and salt. His heart may also be unable to handle the blood returning from his pulmonary system, resulting in a damming of the blood in the lungs and increasing his risk of developing pulmonary edema.
Some patients with congestive heart failure benefit from an implanted pacemaker. A pacemaker rhythmically generates impulses that spread throughout the heart to drive the atria and ventricles. A typical pacemaker monitors the electrical activity of the patient's heart and provides pacing to cause the heart to beat (i.e., depolarize) at a desired rate. Patients having electrical-mechanical dysynchrony may receive benefits from a pacemaker that provides cardiac resynchronization, i.e., that paces both ventricles or both atria to improve synchrony.
A rate-responsive pacemaker adjusts the pacing rate to the changing needs of the patient. For example, a rate-responsive pacemaker may normally pace the patient at sixty beats per minute when the patient is sleeping or at rest. When the patient increases his activity, however, the pacemaker may pace the patient's heart more rapidly to produce a higher heart rate.